testing.md: session.append cannot be stubbed in the kit

[karanb192]

Summary

• Adds one kit rule to references/testing.md: on Claude Code 2.1.288 a test cannot answer session.append beneath the plugins, and says what to do instead.

Details

The engine skips any session.append answer that did not follow a next (its message: "an answer without next (the row is kept; next(e) keeps it)"), and a stub that calls next(e) fails with no implementation for session.append. There is no mock.session. The new bullet tells a builder to keep the rewrite in a pure module tested directly, drive the other hooks through $, and prove the session.append path with a headless claude -p run under the harness config plus a grep of the session .jsonl. Stamped observed, checked 2.1.288, with a recheck trigger. Found while building a redaction mod that hooks session.append.

Test plan

• Read the new bullet under "Kit rules" in references/testing.md
• node --test 'plugins/mod-builder/skills/mod-builder/tests/*.test.mjs' still passes (docs-only change)